The present invention relates to an injection molding machine for producing a molded part by injection of a plasticized material into a two part mold. More particularly, the present invention relates to an improved injection molding machine controller coupled with the hydraulic system for operating the molding machine more efficiently.
Injection molding machines have been increasingly used in the last twenty years for molding a part by injection of a material into a two part mold. An injection molding machine converts a plastic or rubber material from one form or shape to another. Most plastic molding materials initially are in pellet form, but other materials are in strip, powder or webbed form. A molding machine conventionally includes a clamping mechanism including one or more clamping cylinders for applying a mold clamping force to the two part (or multi-part) mold and for moving the mold parts apart to open the mold, and an injection mechanism for plasticizing the material and injecting the plasticized material into the closed mold to form the molded part. The injection mechanism conventionally includes a screw member rotatable and axially moveable within a barrel having a discharge end in fluid communication with the closed mold. The material that enters the molding machine is converted to a molten state partially by heater bands applied to the injection barrel, but primarily by frictional heat generated by the screw rotation. The screw rotation thus changes the solid material into a molten and homogenous mix and transfers the molten material to the front of the injection barrel in preparation for injecting the molten material into a mold once the mold is closed and pressure has built up between the mold halves by the mold clamping mechanism. A hydraulic system is provided for powering the clamping mechanism, and typically also the injection mechanism.
During operation, the moving platen of the molding machine holding the ejection half of the mold closes to contact the injection half of the mold. At this stage, high pressure built up by the clamping mechanism presses the mold halves together. High pressure is built up and a signal is generated by a pressure sensor or limit switch to initiate the injection of the material by the screw. Air is trapped in the cavities of the closed mold that is shut at high pressure. Consequently, the injection speed must be reduced to allow the trapped air to be forced slowly about the ejector pin ports (pin clearance) or through machined vent grooves in the mold. In many applications, even slowing the injection speed will not mold a good part, since the trapped air creates burn marks due to high pressure compressing the trapped air and creating a void or short part, which may also be due to trapped air.
When the injection is completed, the screw rotation will conventionally start at full hydraulic pressure applied to the mold, with the hydraulic motor driving the screw. The screw rotation speed is conventionally controlled by a flow control valve. The screw rotation may end at a preset value established by an electrical switch or by a linear potentiometer. When the screw rotation ends, the screw decompression starts by linear movement or pull back of the screw. The stroke of the pull back may be controlled by a limit switch or a linear potentiometer. At the end of the screw pull back, the clamp may still be closed for a predetermined time to allow for the molded part to cool. When the cooling time ends, the mold opens and the part is ejected. The machine is now ready to start a new cycle.
The operation of a molding machine as described above is conventionally driven by an electric motor coupled to a hydraulic pump or pumps that pump the oil from a tank, deliver the oil to the different actuators of the molding machine, and return the oil to the tank. Most machines use AC motors with fixed volume pumps or a combination of fixed and variable pumps. Both the pumps and the motors run at a fixed speed regardless of the load requirement, which wastes energy.
U.S. Pat. No. 5,756,019 discloses an injection molding machine with a conventional screw member rotatable within a barrel, with a supplying hopper at one end and a port into the closed mold at the other end. The mold clamping force is controlled in response to molding data and position sensors for the screw member and the clamping mechanism during injection. The injection speed is controlled, so that the detected injection pressure gradually increases from the initiation of injection. This detected pressure, the position sensor data, and the molding data are input to a controller which reduces the mold clamping force to a minimum value, thereby reducing power consumption. A closed loop hydraulic system is controlled for operating the screw, and another closed loop hydraulic system is provided for the clamp. Injection molding is the most widely used process for the production of plastic parts. The molding machine ideally produces parts at the lowest possible temperatures, at the lowest applied pressures, in the shortest possible times, and with the lowest energy consumption. Present day molding machines are highly productive, but the molding operation generally faces two significant problems: (1) evacuating the air trapped in the mold cavities in which the mold material is injected, and (2) high energy consumption. Air trapped in mold slows down the injection process, taking longer for a part to fill out. Higher material temperatures and higher mold temperatures are required to keep the material flowing to fill out the part. Higher temperatures in the material and the mold require longer cycle time, thereby requiring more cooling for the mold, higher energy consumption, and longer injection time periods.
U.S. Pat. No. 5,756,019 discloses an injection molding machine with a conventional screw member rotatable within a barrel, with a supplying hopper at one end and a port into the closed mold at the other end. The mold clamping force is controlled in response to molding data and position sensors for the screw member and the clamping mechanism during injection. The injection speed is controlled, so that the detected injection pressure gradually increases from the initiation of injection. This detected pressure, the position sensor data, and the molding data are input to a controller which reduces the mold clamping force to a minimum value, thereby reducing power consumption. A closed loop hydraulic system is controlled for operating the screw, and another closed loop hydraulic system is provided for the clamp. The ""019 patent detects injection pressure at a flow control valve, so that the mold clamping force is controlled, along with other parameters, by the detected injection pressure. Detected pressure may, for example, then be multiplied by a factor to determine the clamping pressure. The position sensor is used to detect the screw position, and another position sensor is used to detect the position of the movable mold. A pressure detector monitors the injection pressure to the screw, while another pressure detector monitors the mold clamp pressure. All of this information is input to a controller, and inherently the system as shown in FIG. 5 of this patent takes a significant amount of time, e.g., in the range from 15 to 50 milliseconds, between a signal being sensed by a sensor and the time the controller operates a molding machine component in response to that sensed signal, and is subject to considerable variations from cycle to cycle. Systems which are based on multiple sensors and perform multiple functions may require a time lag of over 100 milliseconds or more between a sensed signal and a resulting action. Injection molding machines today are able to produce a part of varying size during injection cycle which may take no more than one second, the time delay between sensed signals and the resulting action according to the prior art techniques thus does not allow the injection mold operation to be performed in an efficient manner since the time delay contributes to its inefficient operation. Accordingly, the injection time must be increased and the injection speed reduced in view of the control procedures, which then decreases the efficiency and increased cost.
Injection molding machines have always sought to control machine operation to produce the part faster and thus more efficiently, and also to produce the part using less energy. U.S. Pat. No. 4,988,233 discloses an injection molding machine which uses a brushless DC motor. The patent also discloses a molding machine controller for operating the molding machine during the full injection cycle.
U.S. Pat. No. 6,011,376 discloses an injection mold controller which verifies the linear position of a machine element as detected by a transducer. The controller may be used to detect faulty machine conditions and provide machine malfunction or damage information.
U.S. Pat. No. 5,052,909 discloses a controller designed to conserve energy use of the injection molding machine. The controller outputs driving signals to adjust the speed of the motor so that the flow delivered by the pump substantially matches the hydraulic demand imposed during each phase of the injection. A variable displacement pump is connecting to a fast responding pump control for selectively carrying out pressure compensation or flow compensation. The ""909 Patent discusses the typical operation of a molding machine, listing the phases of the injection cycle which require different hydraulic pressures and/or flow.
Many injection molding machines still use relatively expensive and noisy variable volume pumps, with swatch plates and pump controls which have an adverse effect on the overall efficiency on the injection molding process. During the back-out screw rotation phase of the injection cycle pressure to the screw conventionally is not controlled, and only flow to the screw is controlled with a flow control valve, thereby again wasting valuable energy.
The disadvantages of the prior art are overcome by the present invention, and an improved molding machine and a molding machine controller is provided which may be used to produce molded parts more efficiently, and in most cases using substantially less energy.
An injection molding machine is provided for producing a molded part by injection of a material into a two part mold. A clamping mechanism conventionally includes one or more clamping cylinders for applying a variable mold clamping force to the two part mold, and also for moving the mold parts apart to open the mold. The injection molding machine includes an injection mechanism for plasticizing material and injecting the plasticized material into the closed mold cavity, thereby forming the molded part. The injection mechanism includes a screw member rotatable and axially moveable within a barrel having a discharge end in fluid communication with the closed mold cavity. In one embodiment, the controller coupled to the hydraulic system supplies hydraulic pressure to operate the clamping mechanism, and automatically increases or decreases clamping pressure in response to axial movement of the screw member within the barrel of the injection mechanism, such that air escapes from the mold cavity as a plasticized material flows into the mold. In another embodiment, the controller may directly control one or more variable speed electric motors which drive the injection mechanism and/or the clamping mechanism.
The controller may coupled with a hydraulic system which powers both the clamping mechanism and the injection mechanism, with the controller providing a minimum clamp pressure to maintain plasticized material within the mold cavity as a function of the position of the screw member.
The present invention provides a high performance, low power consumption, hydraulically powered plastic or rubber molding machine, such as an injection molding machine, an injection blow molding machine, an injection stretch blow molding machine, and a structural foam molding machine. The molding machine may incorporate a fast injection speed, a fast air evacuation from the mold, low energy consumption, low molding temperatures, low clamp pressure, low screw rotation pressure, low cycle times, and better quality parts. The molding machine of this invention may intelligently incorporate a PC or a PLC based controls, with either simple or complex algorithms in the controller controlling hydraulic forces to operate the molding machine. Proportional pressure control valves, fixed volume pumps, and variable speed electric motor(s), preferable of the brushless DC rare earth type with sinusoidal wave forms drives, may be used to enhance efficiency and reduce costs. These motors have a smaller frame size, and commutated sinusoidally are highly efficient, generate low heat, and have a smoother and more accurate operation than commercially alternative motors. DC rare earth motors are also built to rapidly accelerate and decelerate, which is highly desirable when the motor control operation of the hydraulically powered components of the injection molding machine.
Fast air evacuation from the mold may be accomplished simultaneously while injecting the plastic material into the mold, and maintaining the minimum clamp pressure applied to the mold halves, both 1) during injection and during the pack and hold portions of the injection cycle, and 2) during the screw recovery portion of the injection cycle. The controller may also minimize energy costs by minimizing screw rotation, torque and speed used to soften the plastic into a homogeneous flowable mass ready to be injected in the mold during the next injection.
It is a feature of the invention that the molding machine is provided with a controller for regulating the hydraulic system which powers the clamping mechanism and/or the injection mechanism, or the electrical drive system which powers the clamping mechanism and/or the injection mechanism. A simple and highly reliable and precise screw axial position sensor may output signals to the controller to automatically increase clamping pressure as the molten material is injected in the mold. The controller may thus anticipate the minimum clamping pressure required to satisfactorily retain the plasticized material in the mold as a function of the position of the screw during an initial injection portion of the cycle. During this initial injection portion of the cycle, a majority of the plasticized material may flow into the mold cavity. Air may be bled through a gap between mating faces of the two part mold, or air may be bled from the gap between the ID of an ejector port and the OD of an ejector pin.
During a final injection portion of the cycle, control of clamping pressure may be a function of time, such that plasticized material preferably packs and solidifies in the two-part mold during the final injection portion of the cycle.
As yet another feature, the controller may regulate hydraulic pressure at different stages of the cycle to axially move the screw, and hydraulic pressure to the motor which rotates the screw, both in response to the axial screw position sensor, to achieve minimum hydraulic pressure to axially move the screw within the time desired, and to achieve the minimum screw rotation rate required to plasticize the material to the characteristics desired for injection into the mold.
Still another feature of the invention is that the controller may set clamp pressure at a selected value higher or lower than hydraulic injection pressure during portions of an injection cycle. The controller may automatically increase and decrease hydraulic pressure during portions of the injection cycle according to preselected values.
A significant advantage of the present invention is that the efficiency of the molding operation may be increased utilizing the controller of the present invention. A related advantage is that less energy is required to mold a part than with prior art equipment.
Another significant advantage of the present invention is that the molding machine may allow an energy management and/or efficient management information to be displayed to the molding machine operator on a chart to enhance the performance and reduce the costs of the molding operation.
Another advantage of the invention is that the controller may power the hydraulic system or the electrical drive system of a molding machine.
These and further objects, features and advantages of the present invention will become apparent from the following detailed description, which makes reference to the figures in the accompanying drawings.